Methods of making fiber waveguides from multilayer structures

ABSTRACT

Methods for making fiber waveguides include rolling a multilayer structure into a spiral structure and forming the fiber waveguide from the spiral structure. The forming includes drawing a fiber preform derived from the spiral structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/733,873, filed on Dec. 10, 2003, now U.S. Pat. No. 7,272,285 which isa continuation-in-part of U.S. application Ser. No. 10/196,403, filed onJul. 16, 2002 now U.S. Pat. No. 7,311,962, which claims priority toProvisional Patent Application No. 60/305,839, filed on Jul. 16, 2001,and to Provisional Patent Application 60/351,066, filed on Jan. 23,2002. U.S. application Ser. No. 10/733,873 also claims priority toProvisional Patent Application 60/432,059, filed on Dec. 10, 2002. Thecontents of each of the abovementioned applications are herebyincorporated by reference in their entirety.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with government support under Grant NumberECS-0123460 awarded by NSF, and Grant Number DAAD19-01-1-0647, awardedby the Army. The government has certain rights in the invention.

BACKGROUND

This invention relates to the field of fiber waveguides and methods formaking waveguides.

Waveguides play important roles in numerous industries. For example,optical waveguides are widely used in telecommunications networks, wherefiber waveguides such as optical fibers are used to carry informationbetween different locations as optical signals. Such waveguidessubstantially confine the optical signals to propagation along apreferred path or paths. Other applications of optical waveguidesinclude imaging applications, such as in an endoscope, and in opticaldetection.

The most prevalent type of fiber waveguide is an optical fiber, whichutilizes index guiding to confine an optical signal to a preferred path.Such fibers include a core region extending along a waveguide axis and acladding region surrounding the core about the waveguide axis and havinga refractive index less than that of the core region. Because of theindex-contrast, optical rays propagating substantially along thewaveguide axis in the higher-index core can undergo total internalreflection (TIR) from the core-cladding interface. As a result, theoptical fiber guides one or more modes of electromagnetic (EM) radiationto propagate in the core along the waveguide axis. The number of suchguided modes increases with core diameter. Notably, the index-guidingmechanism precludes the presence of any cladding modes lying below thelowest-frequency guided mode for a given wavevector parallel to thewaveguide axis. Almost all index-guided optical fibers in usecommercially are silica-based in which one or both of the core andcladding are doped with impurities to produce the index contrast andgenerate the core-cladding interface. For example, commonly used silicaoptical fibers have indices of about 1.45 and index contrasts rangingfrom about 0.2% to 3% for wavelengths in the range of 1.5 microns,depending on the application.

Drawing a fiber from a preform is the most commonly used method formaking fiber waveguides. A preform is a short rod (e.g., 10 to 20 incheslong) having the precise form and composition of the desired fiber. Thediameter of the preform, however, is much larger than the fiber diameter(e.g., 100's to 1000's of times larger). Typically, when drawing anoptical fiber, the material composition of a preform includes a singleglass having varying levels of one or more dopants provided in thepreform core to increase the core's refractive index relative to thecladding refractive index. This ensures that the material forming thecore and cladding are rheologically and chemically similar to be drawn,while still providing sufficient index contrast to support guided modesin the core. To form the fiber from the preform a furnace heats thepreform to a temperature at which the glass viscosity is sufficientlylow (e.g., less than 108 Poise) to draw fiber from the preform. Upondrawing, the preform necks down to a fiber that has the samecross-sectional composition and structure as the preform. The diameterof the fiber is determined by the specific rheological properties of thefiber and the rate at which it is drawn.

Preforms can be made using many techniques known to those skilled in theart, including modified chemical vapor deposition (MCVD), outside vapordeposition (OVD), plasma activated chemical vapor deposition (PCVD) andvapor axial deposition (VAD). Each process typically involves depositinglayers of vaporized raw materials onto a wall of a pre-made tube or rodin the form of soot. Each soot layer is fused shortly after deposition.This results in a preform tube that is subsequently collapsed into asolid rod, over jacketed, and then drawn into fiber.

Optical fibers applications can be limited by wavelength and signalpower. Preferably, fibers should be formed from materials that have lowabsorption of energy at guided wavelengths and should have minimaldefects. Where absorption is high, it can reduce signal strength tolevels indistinguishable from noise for transmission over long fibers.Even for relatively low absorption materials, absorption by the coreand/or cladding heats the fiber. Defects can scatter guided radiationout of the core, which can also lead to heating of the fiber. Above acertain power density, this heating can irreparably damage the fiber.Accordingly, many applications that utilize high power radiation sourcesuse apparatus other than optical fibers to guide the radiation from thesource to its destination.

SUMMARY

In certain aspects, the invention features photonic crystal waveguides(e.g., Bragg fibers) that include polymer portions and glass portions(e.g., chalcogenide glass portions). In some embodiments, the photoniccrystal waveguides include a hollow core. Confinement of radiation inthe hollow core is provided by photonic bandgaps established by multiplealternating layers of polymer and glass (e.g., a continuous polymerlayer and a continuous glass layer wound into a spiral). Typically, theglass layers have a high refractive index and the polymer layers have alow refractive index. Fundamental and high-order spectral transmissionwindows are determined by optical thickness of the alternating layersand can be scaled from visible radiation (e.g., having wavelengths 0.35to 0.75 microns) to infrared radiation (e.g., having wavelengths 0.75 toabout 15 microns or more).

The invention also features methods for making fiber photonic crystalfiber waveguides. A polymer substrate is coated with a layer of glass toform a planar multilayer film. The multilayer film is then rolled toprovide a hollow multilayer tube with a spiral cross section. The hollowtube is subsequently consolidated by heating to fuse the spiral layersand provide a hollow fiber preform, which is drawn into the fiberwaveguide.

Materials can be selected to provide high index contrast betweendifferent portions of the photonic crystal waveguides. High indexcontrast can provide fibers with large photonic bandgaps andomnidirectional reflectivity. The large photonic bandgaps can result inshort penetration depths within portions of the waveguide surroundingthe core, reducing radiation and absorption losses of radiation guidedby the fiber.

Co-drawing optically dissimilar materials can provide fibers with lowdefect densities when the thermo-mechanical, rheological, andphysico-chemical properties of the fiber materials are compatible.Accordingly, in certain aspects, the invention features combinations ofglasses and polymers which can be co-drawn and criteria for selectingglasses and polymers that can be co-drawn.

Low loss, low defect density fibers can be used to guide high powerradiation with little or no damage to the fiber.

In general, in a first aspect, the invention features a method includingrolling a multilayer structure into a spiral structure, and forming afiber waveguide, wherein the forming includes drawing a fiber preformderived from the spiral structure.

Embodiments of the methods can include one or more of the followingfeatures and/or feature of other aspects.

The multilayer structure can include at least two layers comprisingmaterials with different refractive indices. The layers can include alayer of a first material and a pair of layers of a second materialsandwiching the first material layer. The layers can be substantiallyplanar. The different materials can include a first material thatincludes a glass and a second material that includes a polymer. In someembodiments, the different materials include a high-index material and alow-index material, and wherein a ratio of the refractive index of thehigh-index material to that of the low-index material is greater than1.5 (e.g., greater than 1.8).

The method can further include disposing at least a first layer of afirst material (e.g., a glass, such as a chalcogenide glass) on a secondlayer of a second material (e.g., a polymer, such as PES or PEI)different from that of the first material to form the multilayerstructure. The first material can be disposed on both sides of thesecond layer. The disposing can include sputtering or evaporating.Additional layers can be disposed on the first and second layers to formthe multilayer article.

The multilayer structure can be rolled around a rod (e.g., a hollow rod)to form the spiral structure. The method can include consolidating thespiral structure to form the preform. Consolidating can include heatingthe spiral structure. In some embodiments, consolidating includesheating the spiral structure under vacuum. The method can includeremoving the rod (e.g., by chemical etching) from the preform prior tothe drawing.

The spiral structure can include a core surrounded by alternating layersof the multilayer structure. The fiber waveguide can include a hollowcore surrounded by multiple layers corresponding to the multilayerstructure.

In general, in another aspect, the invention features an articleincluding a fiber waveguide that includes alternating layers ofdifferent materials surrounding a core extending along a waveguide axis,wherein the alternating layers define a spiral structure.

Embodiments of the article can include one or more of the followingfeatures and/or features of other aspects.

The spiral structure can include a multilayer structure including atleast two layers of the different materials encircling the core multipletimes. The different materials can include a high-index dielectricmaterial and a low-index dielectric material, and wherein a ratio of therefractive index of the high-index material to that of the low-indexmaterial is greater than 1.5 (e.g., greater than 1.8). The differentmaterials can include a polymer (e.g., PES) and a chalcogenide glass(e.g., As₂Se₃).

The inner most layer of the alternating layers can have a thicknesssmaller than that of subsequent layers of the same material. Thicknessesof the alternating layers can be selected to guide EM radiation alongthe waveguide axis in at a wavelength in the range of about 8-12 microns(e.g., at a wavelength of about 10.6 microns). In some embodiments,thicknesses of the alternating layers are selected to guide EM radiationalong the waveguide axis at a wavelength in the range of about 2-5microns.

The core can be hollow. The fiber waveguide can exhibit transmissionlosses smaller than about 1 dB/m at a selected wavelength for a straightlength of the fiber (e.g., for a wavelength in a range of about 0.75 toabout 10.6 microns). In some embodiments, the selected wavelength isabout 10.6 microns.

The fiber waveguide can exhibit transmission losses smaller than about1.5 dB at a selected wavelength when bent around a 90 degree turn withany bending radius within a range of about 4-10 cm (e.g., for awavelength in a range of about 0.75 to about 10.6 microns).

The fiber waveguide can be capable of guiding EM radiation along thewaveguide axis at power densities greater than or equal to about 300W/cm² for a selected wavelength (e.g., for a wavelength in a range ofabout 0.75 to about 10.6 microns). In some embodiments, the selectedwavelength is about 10.6 microns. The fiber waveguide can be capable ofguiding the EM radiation along the waveguide axis at power densitiesgreater than or equal to about 300 W/cm² for the selected wavelengtheven when the fiber waveguide is smoothly bent around a 90 degree turnwith a bent length of at least 0.3 m.

The fiber waveguide can be capable of guiding the EM radiation along thewaveguide axis at powers greater than or equal to about 25 W for aselected wavelength (e.g., for a wavelength in a range of about 0.75 toabout 10.6 microns). In some embodiments, the selected wavelength isabout 10.6 microns.

In general, in a further aspect, the invention features an articleincluding a high-power, low-loss fiber waveguide that includesalternating layers of different dielectric materials surrounding a coreextending along a waveguide axis, the different dielectric materialsincluding a polymer and a glass.

Embodiments of the article can include one or more of the followingfeatures and/or features of other aspects.

The alternating layers can define a spiral structure. The spiralstructure can include a multilayer structure comprising at least twolayers of the different materials encircling the core multiple times.The different materials can include a high-index dielectric material anda low-index dielectric material, and wherein a ratio of the refractiveindex of the high-index material to that of the low-index material isgreater than 1.5. The different materials can include a high-indexdielectric material and a low-index dielectric material, and wherein aratio of the refractive index of the high-index material to that of thelow-index material is greater than 1.8. The glass can include achalcogenide glass (e.g., As₂Se₃). The polymer can include PES or PEI.The inner most layer of the alternating layers can have a thicknesssmaller than that of subsequent layers of the same material. Thicknessesof the alternating layers can be selected to guide EM radiation alongthe waveguide axis at a wavelength in the range of about 8-12 microns(e.g., at a wavelength of about 10.6 microns). In some embodiments,thicknesses of the alternating layers are selected to guide EM radiationalong the waveguide axis at a wavelength in the range of about 2-5microns.

The core can be hollow.

The fiber waveguide can exhibit transmission losses smaller than about 1dB/m at a selected wavelength for a straight length of the fiberwaveguide (e.g., for a wavelength in a range of about 0.75 to about 10.6microns). The selected wavelength can be about 10.6 microns.

The fiber waveguide can exhibit transmission losses smaller than about1.5 dB at the selected wavelength when bent around a 90 degree turn withany bending radius within a range of about 4-10 cm (e.g., for awavelength in a range of about 0.75 to about 10.6 microns). The selectedwavelength can be about 10.6 microns.

The fiber waveguide can be capable of guiding EM radiation along thewaveguide axis at power densities greater than or equal to about 300W/cm² for a selected wavelength (e.g., for a wavelength in a range ofabout 0.75 to about 10.6 microns). The selected wavelength can be about10.6 microns.

The fiber waveguide can be capable of guiding the EM radiation along thewaveguide axis at power densities greater than or equal to about 300W/cm² for the selected wavelength even when the fiber waveguide issmoothly bent around a 90 degree turn with a bent length of at least 0.3m.

The fiber waveguide can be capable of guiding the EM radiation along thewaveguide axis at powers greater than or equal to about 25 W for aselected wavelength (e.g., for a wavelength in a range of about 0.75 toabout 10.6 microns). The selected wavelength can be about 10.6 microns.

Embodiments of the invention may have one or more of the followingadvantages.

Photonic crystal fiber waveguides can have low transmission loss, forboth straight and bent lengths of fiber. They may be used to guide highpower EM radiation. They can be used to guide EM radiation with highpower densities. They may be used to guide EM radiation at IRwavelengths (e.g., 0.75 to about 12 microns or more).

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1A is a cross-sectional view of an embodiment of a photonic crystalfiber waveguide.

FIG. 1B is a plot of the refractive index profile of a part of thephotonic crystal fiber waveguide shown in FIG. 1A.

FIG. 2A-2D are schematic diagrams showing steps in a method for making aphotonic crystal fiber waveguide.

FIG. 3A is a cross-sectional view of a confinement region of anembodiment of a photonic crystal fiber waveguide.

FIG. 3B is a plot of the refractive index profile of the confinementregion shown in FIG. 3A.

FIGS. 4A and 4B are SEM micrographs of an example of a photonic crystalfiber waveguide.

FIG. 5 are plots of transmission spectra for two different examples ofphotonic crystal fiber waveguides. The tallest transmission peakcorresponds to the primary photonic band gap in each case, while thearrows indicate higher order band gaps.

FIG. 6A is a plot of the transmission spectrum of an example of aphotonic crystal fiber waveguide.

FIG. 6B is a plot of the log of transmitted power as a function of fiberlength for an example of a photonic crystal fiber waveguide cut todifferent lengths. The slope of the plot is in dB/m.

FIG. 7 is a plot showing transmission spectra for a fiber bent withvarying radii of curvature.

FIG. 8 is a plot of bend loss as a function of bend curvature fortransmission of EM radiation from a CO2 laser through an photoniccrystal fiber waveguide. Loss values were obtained by comparing thetotal transmitted power through the bent fiber to the same fiber whenheld straight.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIG. 1A, a photonic crystal fiber waveguide 100 includes acore 120 extending along a waveguide axis and a dielectric confinementregion 110 (e.g., alternating high index and low index layers)surrounding the core. Confinement region 110 is surrounded by a supportlayer 150, which provides mechanical support for the confinement region.

Confinement region 110 includes continuous layers 130 and 140 ofdielectric material (e.g., polymer, glass) having different refractiveindices, as opposed to multiple discrete, concentric layers that formconfinement regions in other embodiments. Continuous layers 130 and 140form a spiral around an axis 199 along which the photonic crystal fiberwaveguide guides electromagnetic radiation. One of the layers, e.g.,layer 140, is a high-index layer having an index n_(H) and a thicknessd_(H), and the layer, e.g., layer 130, is a low-index layer having anindex n_(L) and a thickness d_(L), where n_(H)>n_(L) (e.g., n_(H)−n_(L)can be greater than or equal to or greater than 0.01, 0.05, 0.1, 0.2,0.5 or more). Because layers 130 and 140 spiral around axis 199, aradial section 160 extending from axis 199 intersects each of the layersmore than once, providing a radial profile that includes alternatinghigh index and low index layers.

Referring to FIG. 1B, optically, the spiraled layers provide a periodicvariation in the index of refraction along radial section 160, with aperiod corresponding to the optical thickness of layer 130 and layer140, i.e., confinement region 110 has an bilayer optical periodn_(H)d_(H)+n_(L)d_(L). “R” refers to the radial position as measuredfrom axis 199.

The thickness (d_(H) and d_(L)) and optical thickness (n_(H)d_(H) andn_(L)d_(L)) of layers 130 and 140 can vary. In some embodiments, theoptical thickness of layer 130 and layer 140 are the same. Layerthickness is usually selected based on the desired optical performanceof the fiber (e.g., according to the wavelength radiation to be guided).The relationship between layer thickness and optical performance isdiscussed below. Typically, layer thickness is in the sub-micron to tensof micron range. For example, layers 130 and 140 can be between about0.1 μm to 20 μm thick (e.g, about 0.5 to 5 μm thick).

For the embodiment shown in FIG. 1A, confinement region 110 is 5bilayers thick. In practice, however, confinement region 110 may includemany more bilayers (e.g., more than about 8 bilayers, 10 bilayers, 15bilayers, 20 bilayers, 25 bilayers, such as 40 or more bilayers).

Layer 140 includes a material that has a high refractive index, such asa chalcogenide glass. Layer 130 includes a material having a refractiveindex lower than the high index material of layer 140, and is typicallymechanically flexible. For example, layer 130 often includes a polymer.Preferably, the materials forming layer 130 and layer 140 can beco-drawn. Criteria for selecting materials that can be co-drawn arediscussed below.

In the present embodiment, core 120 is hollow. Optionally, the hollowcore can be filled with a fluid, such as a gas (e.g., air, nitrogen,and/or a noble gas) or liquid (e.g., an isotropic liquid or a liquidcrystal). Alternatively, core 120 can include any material orcombination of materials that are rheologically compatible with thematerials forming confinement region 110. In certain embodiments, core120 can include one or more dopant materials, such as those described inU.S. patent application Ser. No. 10/121,452, entitled “HIGHINDEX-CONTRAST FIBER WAVEGUIDES AND APPLICATIONS,” filed Apr. 12, 2002and now published under Pub. No. US-2003-0044158-A1, the entire contentsof which are hereby incorporated by reference.

Core and confinement regions 120 and 110 may include multiple dielectricmaterials having different refractive indices. In such cases, we mayrefer to an “average refractive index” of a given region, which refersto the sum of the weighted indices for the constituents of the region,where each index is weighted by the fractional area in the region of itsconstituent. The boundary between layers 130 and 140, however, isdefined by a change in index. The change may be caused by the interfaceof two different dielectric materials or by different dopantconcentrations in the same dielectric material (e.g., different dopantconcentrations in silica).

Dielectric confinement region 110 guides EM radiation in a first rangeof wavelengths to propagate in dielectric core 120 along waveguide axis199. The confinement mechanism is based on a photonic crystal structurein region 110 that forms a bandgap including the first range ofwavelengths. Because the confinement mechanism is not index-guiding, itis not necessary for the core to have a higher index than that of theportion of the confinement region immediately adjacent the core. To thecontrary, core 120 may have a lower average index than that ofconfinement region 110. For example, core 120 may be air, some othergas, such as nitrogen, or substantially evacuated. In such a case, EMradiation guided in the core will have much smaller losses and muchsmaller nonlinear interactions than EM radiation guided in a silicacore, reflecting the smaller absorption and nonlinear interactionconstants of many gases relative to silica or other such solid material.In additional embodiments, for example, core 120 may include a porousdielectric material to provide some structural support for thesurrounding confinement region while still defining a core that islargely air. Accordingly, core 120 need not have a uniform indexprofile.

Layers 130 and 140 of confinement region 110 form what is known as aBragg fiber. The periodic optical structure of the spirally wound layersare analogous to the alternating layers of a planar dielectric stackreflector (which is also known as a Bragg mirror). The layers ofconfinement region 110 and the alternating planar layers of a dielectricstack reflector are both examples of a photonic crystal structure.Photonic crystal structures are described generally in Photonic Crystalsby John D. Joannopoulos et al. (Princeton University Press, PrincetonN.J., 1995).

As used herein, a photonic crystal is a dielectric structure with arefractive index modulation that produces a photonic bandgap in thephotonic crystal. A photonic bandgap, as used herein, is a range ofwavelengths (or inversely, frequencies) in which there are no accessibleextended (i.e., propagating, non-localized) states in the dielectricstructure. Typically the structure is a periodic dielectric structure,but it may also include, e.g., more complex “quasi-crystals.” Thebandgap can be used to confine, guide, and/or localize light bycombining the photonic crystal with “defect” regions that deviate fromthe bandgap structure. Moreover, there are accessible extended statesfor wavelengths both below and above the gap, allowing light to beconfined even in lower-index regions (in contrast to index-guided TIRstructures, such as those described above). The term “accessible” statesmeans those states with which coupling is not already forbidden by somesymmetry or conservation law of the system. For example, intwo-dimensional systems, polarization is conserved, so only states of asimilar polarization need to be excluded from the bandgap. In awaveguide with uniform cross-section (such as a typical fiber), thewavevector β is conserved, so only states with a given β need to beexcluded from the bandgap to support photonic crystal guided modes.Moreover, in a waveguide with cylindrical symmetry, the “angularmomentum” index m is conserved, so only modes with the same m need to beexcluded from the bandgap. In short, for high-symmetry systems therequirements for photonic bandgaps are considerably relaxed compared to“complete” bandgaps in which all states, regardless of symmetry, areexcluded.

Accordingly, the dielectric stack reflector is highly reflective in thephotonic bandgap because EM radiation cannot propagate through thestack. Similarly, the layers in confinement region 110 provideconfinement because they are highly reflective for incident rays in thebandgap. Strictly speaking, a photonic crystal is only completelyreflective in the bandgap when the index modulation in the photoniccrystal has an infinite extent. Otherwise, incident radiation can“tunnel” through the photonic crystal via an evanescent mode thatcouples propagating modes on either side of the photonic crystal. Inpractice, however, the rate of such tunneling decreases exponentiallywith photonic crystal thickness (e.g., the number of alternatinglayers). It also decreases with the magnitude of the index-contrast inthe confinement region.

Furthermore, a photonic bandgap may extend over only a relatively smallregion of propagation vectors. For example, a dielectric stack may behighly reflective for a normally incident ray and yet only partiallyreflective for an obliquely incident ray. A “complete photonic bandgap”is a bandgap that extends over all possible wavevectors and allpolarizations. Generally, a complete photonic bandgap is only associatedwith a photonic crystal having index modulations along three dimensions.However, in the context of EM radiation incident on a photonic crystalfrom an adjacent dielectric material, we can also define an“omnidirectional photonic bandgap,” which is a photonic bandgap for allpossible wavevectors and polarizations for which the adjacent dielectricmaterial supports propagating EM modes. Equivalently, an omnidirectionalphotonic bandgap can be defined as a photonic band gap for all EM modesabove the light line, wherein the light line defines the lowestfrequency propagating mode supported by the material adjacent thephotonic crystal. For example, in air the light line is approximatelygiven by ω=cβ, where ω is the angular frequency of the radiation, β isthe wavevector, and c is the speed of light. A description of anomnidirectional planar reflector is disclosed in U.S. Pat. No.6,130,780, the contents of which are incorporated herein by reference.Furthermore, the use of alternating dielectric layers to provideomnidirectional reflection (in a planar limit) for a cylindricalwaveguide geometry is disclosed in U.S. Pat. No. 6,463,200, entitled“OMNIDIRECTIONAL MULTILAYER DEVICE FOR ENHANCED OPTICAL WAVEGUIDING,” toYoel Fink et al., the contents of which are incorporated herein byreference.

When alternating layers 130 and 140 in confinement region 110 give riseto an omnidirectional bandgap with respect to core 120, the guided modesare strongly confined because, in principle, any EM radiation incidenton the confinement region from the core is completely reflected.However, such complete reflection only occurs when there are an infinitenumber of layers. For a finite number of layers (e.g., about 10bilayers), an omnidirectional photonic bandgap may correspond to areflection in a planar geometry of at least 95% for all angles ofincidence ranging from 0° to 80° and for all polarizations of EMradiation having frequency in the omnidirectional bandgap. Furthermore,even when photonic crystal fiber waveguide 100 has a confinement regionwith a bandgap that is not omnidirectional, it may still support astrongly guided mode, e.g., a mode with radiation losses of less than0.1 dB/km for a range of frequencies in the bandgap. Generally, whetheror not the bandgap is omnidirectional will depend on the size of thebandgap produced by the alternating layer (which generally scales withindex-contrast of the two layers) and the lowest-index constituent ofthe photonic crystal.

In a Bragg-like configuration, the high-index layers may vary in indexand thickness, and/or the low-index layers may vary in index andthickness. The confinement region may also include a periodic structureincluding more than two layers per period (e.g., three or more layersper period). Moreover, the refractive index modulation may varycontinuously or discontinuously as a function of fiber radius within theconfinement region. In general, the confinement region may be based onany index modulation that creates a photonic bandgap.

In the present embodiment, multilayer structure 110 forms a Braggreflector because it has a periodic index variation with respect to theradial axis. A suitable index variation is an approximate quarter-wavecondition. It is well-known that, for normal incidence, a maximum bandgap is obtained for a “quarter-wave” stack in which each layer has equaloptical thickness λ/4, or equivalently d_(H)/d_(L)=n_(L)/n_(H), where dand n refer to the thickness and index, respectively, of the high-indexand low-index layers. These correspond to layers 240 and 230,respectively. Normal incidence corresponds to β=0. For a cylindricalwaveguide, the desired modes typically lie near the light line ω=cβ (inthe large core radius limit, the lowest-order modes are essentiallyplane waves propagating along z-axis, i.e., the waveguide axis). In thiscase, the quarter-wave condition becomes:

$\frac{\mathbb{d}_{H}}{\mathbb{d}_{L}} = \sqrt{\frac{n_{L}^{2} - 1}{n_{H}^{2} - 1}}$

Strictly speaking, this equation may not be exactly optimal because thequarter-wave condition is modified by the cylindrical geometry, whichmay require the optical thickness of each layer to vary smoothly withits radial coordinate. Nonetheless, we find that this equation providesan excellent guideline for optimizing many desirable properties,especially for core radii larger than the mid-bandgap wavelength.

Some embodiments of photonic crystal fiber waveguides are described inU.S. patent application Ser. No. 10/057,258, entitled “LOW-LOSS PHOTONICCRYSTAL FIBER HAVING LARGE CORE RADIUS,” to Steven G. Johnson et al.,filed Jan. 25, 2002 and published under Pub. No. US-2002-0164137-A1, theentire contents of which are hereby incorporated by reference.

The radius of core 120 can vary depending on the end-use application offiber 120. The core radius can depend on the wavelength or wavelengthrange of the energy to be guided by the fiber, and on whether the fiberis a single or multimode fiber. For example, where the fiber is a singlemode fiber for guiding visible wavelengths (e.g., between about 400 nmand 800 nm) the core radius can be in the sub-micron to several micronrange (e.g., from about 0.5 μm to 5 μm). However, where the fiber is amultimode fiber for guiding IR wavelengths (e.g., from about 2 μm to 15μm, such as 10.6 μm), the core radius can be in the tens to thousands ofmicrons range (e.g., from about 10 μm to 2,000 μm, such as 500 μm to1,000 μm). The core radius can be greater than about 5λ (e.g., more thanabout 10λ, 20λ, 30λ, 50λ, 100λ), where λ is the wavelength of the guidedenergy.

As discussed previously, support layer 150 provides mechanical supportfor confinement region 110. The thickness of support layer 150 can varyas desired. In some embodiments, support layer 150 is substantiallythicker than confinement region 110. For example, support layer 150 canbe about 10 or more times thicker than confinement region 110 (e.g.,more than 20, 30, 50 times thicker).

The composition of support layer 150 is usually selected to provide thedesired mechanical support and protection for confinement region 110. Inmany embodiments, support layer 150 is formed from materials that can beco-drawn with the confinement region 110. Criteria for selectingmaterials suitable for co-drawing are discussed below. In someembodiments, support layer can be formed from the same material(s) asused to form confinement region 110. For example, where layer 130 isformed from a polymer, support layer 150 can be formed from the samepolymer.

Turning now to the composition of layers 130 and 140 in confinementregion 110, materials with a suitably high index of refraction to form ahigh index portion (e.g., layer 140) include chalcogenide glasses (e.g.,glasses containing a chalcogen element, such as sulphur, selenium,and/or tellurium), heavy metal oxide glasses, amorphous alloys, andcombinations thereof.

In addition to a chalcogen element, chalcogenide glasses may include oneor more of the following elements: boron, aluminum, silicon, phosphorus,sulfur, gallium, germanium, arsenic, indium, tin, antimony, thallium,lead, bismuth, cadmium, lanthanum and the halides (fluorine, chlorine,bromide, iodine).

Chalcogenide glasses can be binary or ternary glasses, e.g., As—S,As—Se, Ge—S, Ge—Se, As—Te, Sb—Se, As—S—Se, S—Se—Te, As—Se—Te, As—S—Te,Ge—S—Te, Ge—Se—Te, Ge—S—Se, As—Ge—Se, As—Ge—Te, As—Se—Pb, As—S—Tl,As—Se—Tl, As—Te—Tl, As—Se—Ga, Ga—La—S, Ge—Sb—Se or complex,multi-component glasses based on these elements such as As—Ga—Ge—S,Pb—Ga—Ge—S, etc. The ratio of each element in a chalcogenide glass canbe varied. For example, a chalcogenide glass with a suitably highrefractive index may be formed with 5-30 mole % Arsenic, 20-40 mole %Germanium, and 30-60 mole % Selenium.

Examples of heavy metal oxide glasses with high refractive indicesinclude Bi₂O₃—, PbO—, Tl₂O₃—, Ta₂O₃—, TiO₂—, and TeO₂— containingglasses.

Amorphous alloys with suitably high indices of refraction include Al—Te,R—Te(Se) (R=alkali).

Materials with suitably low index of refraction to form a low-indexportion (e.g., layer 130) include oxide glasses, halide glasses,polymers, and combinations thereof. Polymers including those in thecarbonate—(e.g., polycarbonate (PC)), sulfone—(e.g., poly(ethersulphone) (PES)), etherimid—(e.g., poly(ether imide) (PEI)), andacrylate—(e.g., poly(methyl methacrylate) (PMMA)) families as well asfluoropolymers are good matching candidates too.

Suitable oxide glasses may include glasses that contain one or more ofthe following compounds: 0-40 mole % of M₂O where M is Li, Na, K, Rb, orCs; 0-40 mole % of M′O where M′ is Mg, Ca, Sr, Ba, Zn, or Pb; 0-40 mole% of M″₂O₃ where M″ is B, Al, Ga, In, Sn, or Bi; 0-60 mole % P₂O₅; and0-40 mole % SiO₂.

Portions of photonic crystal fiber waveguides can optionally includeother materials. For example, any portion can include one or morematerials that change the index of refraction of the portion. A portioncan include a material that increases the refractive index of theportion. Such materials include, for example, germanium oxide, which canincrease the refractive index of a portion containing a borosilicateglass. Alternatively, a portion can include a material that decreasesthe refractive index of the portion. For example, boron oxide candecrease the refractive index of a portion containing a borosilicateglass.

Portions of high index-contrast fiber waveguides can be homogeneous orinhomogeneous. For example, one or more portions can includenano-particles (e.g., particles sufficiently small to minimally scatterlight at guided wavelengths) of one material embedded in a host materialto form an inhomogeneous portion. An example of this is a high-indexpolymer composite formed by embedding a high-index chalcogenide glassnano-particles in a polymer host. Further examples include CdSe and orPbSe nano-particles in an inorganic glass matrix.

Portions of fiber waveguides can include materials that alter themechanical, rheological and/or thermodynamic behavior of those portionsof the fiber. For example, one or more of the portions can include aplasticizer. Portions may include materials that suppresscrystallization, or other undesirable phase behavior within the fiber.For example, crystallization in polymers may be suppressed by includinga cross-linking agent (e.g., a photosensitive cross-linking agent). Inother examples, if a glass-ceramic material was desired, a nucleatingagent, such as TiO₂ or ZrO₂, can be included in the material.

Portions can also include compounds designed to affect the interfacebetween adjacent portions in the fiber (e.g., between the low index andhigh index layers). Such compounds include adhesion promoters andcompatibilizers. For example, an organo-silane compound can be used topromote adhesion between a silica-based glass portion and a polymerportion. For example, phosphorus or P₂O₅ is compatible with bothchalcogenide and oxide glasses, and may promote adhesion betweenportions formed from these glasses.

Fiber waveguides can include additional materials specific to particularfiber waveguide applications. In fiber amplifiers, for example, any ofthe portions can be formed of any dopant or combination of dopantscapable of interacting with an optical signal in the fiber to enhanceabsorption or emission of one or more wavelengths of light by the fiber,e.g., at least one rare earth ion, such as erbium ions, ytterbium ionsneodymium ions, holmium ions, dysprosium ions, and/or thulium ions.

Portions of high index-contrast waveguides can include one or morenonlinear materials. Nonlinear materials are materials that enhance thenonlinear response of the waveguide. In particular, nonlinear materialshave a larger nonlinear response than silica. For example, nonlinearmaterials have a Kerr nonlinear index, n⁽²⁾, larger than the Kerrnonlinear index of silica (i.e., greater than 3.5×10⁻²⁰ m²/W, such asgreater than 5×10⁻²⁰ m²/W, greater than 10×10⁻²⁰ m²/W, greater than20×10⁻²⁰ m²/W, greater than 100×10⁻²⁰ m²/W, greater than 200×10⁻²⁰m²/W).

When making a robust fiber waveguides using a drawing process, not everycombination of materials with desired optical properties is necessarilysuitable. Typically, one should select materials that are rheologically,thermo-mechanically, and physico-chemically compatible. Several criteriafor selecting compatible materials will now be discussed.

A first criterion is to select materials that are rheologicallycompatible. In other words, one should select materials that havesimilar viscosities over a broad temperature range, corresponding to thetemperatures experience during the different stages of fiber drawing andoperation. Viscosity is the resistance of a fluid to flow under anapplied shear stress. Here, viscosities are quoted in units of Poise.Before elaborating on rheological compatibility, it is usefule define aset of characteristic temperatures for a given material, which aretemperatures at which the given material has a specific viscosity.

The annealing point, T_(a), is the temperature at which a material has aviscosity 10¹³ Poise. T_(a) can be measured using a Model SP-2A Systemfrom Orton Ceramic Foundation (Westerville, Ohio). Typically, T_(a) isthe temperature at which the viscosity of a piece of glass is low enoughto allow for relief of residual stresses.

The softening point, T_(s), is the temperature at which a material has aviscosity 10^(7.65) Poise. T_(s) can be measured using a softening pointinstrument, e.g., Model SP-3A from Orton Ceramic Foundation(Westerville, Ohio). The softening point is related to the temperatureat which the materials flow changes from plastic to viscous in nature.

The working point, T_(w), is the temperature at which a material has aviscosity 10⁴ Poise. T_(w) can be measured using a glass viscometer,e.g., Model SP-4A from Orton Ceramic Foundation (Westerville, Ohio). Theworking point is related to the temperature at which a glass can beeasily drawn into a fiber. In some embodiments, for example, where thematerial is an inorganic glass, the material's working point temperaturecan be greater than 250° C., such as about 300° C., 400° C., 500° C. ormore.

The melting point, T_(m), is the temperature at which a material has aviscosity 10² Poise. T_(m) can also be measured using a glassviscometer, e.g., Model SP-4A from Orton Ceramic Foundation(Westerville, Ohio). The melting point is related to the temperature atwhich a glass becomes a liquid and control of the fiber drawing processwith respect to geometrical maintenance of the fiber becomes verydifficult.

To be rheologically compatible, two materials should have similarviscosities over a broad temperature range, e.g., from the temperatureat which the fiber is drawn down to the temperature at which the fibercan no longer release stress at a discernible rates (e.g., at T_(a)) orlower. Accordingly, the working temperature of two compatible materialsshould be similar, so that the two materials flow at similar rates whendrawn. For example, if one measures the viscosity of the first material,η₁(T) at the working temperature of the second material, T_(w2),η₁(T_(w2)) should be at least 10³ Poise, e.g., 10⁴ Poise or 10⁵ Poise,and no more than 10⁶ Poise. Moreover, as the drawn fiber cools thebehavior of both materials should change from viscous to elastic atsimilar temperatures. In other words, the softening temperature of thetwo materials should be similar. For example, at the softeningtemperature of the second material, T_(s2), the viscosity of the firstmaterial, η₁(T_(s2)) should be at least 10⁶ Poise, e.g., 10⁷ Poise or10⁸ Poise and no more than 10⁹ Poise. In preferred embodiments, itshould be possible to anneal both materials together, so at theannealing temperature of the second material, T_(a2), the viscosity ofthe first material, η₁(T_(a2)) should be at least 10⁸ Poise (e.g., atleast 10⁹ Poise, at least 10¹⁰ Poise, at least 10¹¹ Poise, at least 10¹²Poise, at least 10¹³ Poise, at least 10¹⁴ Poise).

Additionally, to be rheologically compatible, the change in viscosity asa function of temperature (i.e., the viscosity slope) for both materialsshould preferably match as close as possible.

A second selection criterion is that the thermal expansion coefficients(TEC) of each material should be similar at temperatures between theannealing temperatures and room temperature. In other words, as thefiber cools and its rheology changes from liquid-like to solid-like,both materials' volume should change by similar amounts. If the twomaterials TEC's are not sufficiently matched, a large differentialvolume change between two fiber portions can result in a large amount ofresidual stress buildup, which can cause one or more portions to crackand/or delaminate. Residual stress may also cause delayed fracture evenat stresses well below the material's fracture stress.

The TEC is a measure of the fractional change in sample length with achange in temperature. This parameter can be calculated for a givenmaterial from the slope of a temperature-length (or equivalently,temperature-volume) curve. The temperature-length curve of a materialcan be measured using e.g., a dilatometer, such as a Model 1200Ddilatometer from Orton Ceramic Foundation (Westerville, Ohio). The TECcan be measured either over a chosen temperature range or as theinstantaneous change at a given temperature. This quantity has the units° C.⁻¹.

For many materials, there are two linear regions in thetemperature-length curve that have different slopes. There is atransition region where the curve changes from the first to the secondlinear region. This region is associated with a glass transition, wherethe behavior of a glass sample transitions from that normally associatedwith a solid material to that normally associated with a viscous fluid.This is a continuous transition and is characterized by a gradual changein the slope of the temperature-volume curve as opposed to adiscontinuous change in slope. A glass transition temperature, T_(g),can be defined as the temperature at which the extrapolated glass solidand viscous fluid lines intersect. The glass transition temperature is atemperature associated with a change in the materials rheology from abrittle solid to a solid that can flow. Physically, the glass transitiontemperature is related to the thermal energy required to excite variousmolecular translational and rotational modes in the material. The glasstransition temperature is often taken as the approximate annealingpoint, where the viscosity is 10¹³ Poise, but in fact, the measuredT_(g) is a relative value and is dependent upon the measurementtechnique.

A dilatometer can also be used to measure a dilatometric softeningpoint, T_(ds). A dilatometer works by exerting a small compressive loadon a sample and heating the sample. When the sample temperature becomessufficiently high, the material starts to soften and the compressiveload causes a deflection in the sample, when is observed as a decreasein volume or length. This relative value is called the dilatometricsoftening point and usually occurs when the materials viscosity isbetween 10¹⁰ and 10^(12.5) Poise. The exact T_(ds) value for a materialis usually dependent upon the instrument and measurement parameters.When similar instruments and measurement parameters are used, thistemperature provides a useful measure of different materials rheologicalcompatibility in this viscosity regime.

As mentioned above, matching the TEC is an important consideration forobtaining fiber that is free from excessive residual stress, which candevelop in the fiber during the draw process. Typically, when the TEC'sof the two materials are not sufficiently matched, residual stressarises as elastic stress. The elastic stress component stems from thedifference in volume contraction between different materials in thefiber as it cools from the glass transition temperature to roomtemperature (e.g., 25° C.). The volume change is determined by the TECand the change in temperature. For embodiments in which the materials inthe fiber become fused or bonded at any interface during the drawprocess, a difference in their respective TEC's will result in stress atthe interface. One material will be in tension (positive stress) and theother in compression (negative stress), so that the total stress iszero. Moderate compressive stresses themselves are not usually a majorconcern for glass fibers, but tensile stresses are undesirable and maylead to failure over time. Hence, it is desirable to minimize thedifference in TEC's of component materials to minimize elastic stressgeneration in a fiber during drawing. For example, in a composite fiberformed from two different materials, the absolute difference between theTEC's of each glass between T_(g) and room temperature measured with adilatometer with a heating rate of 3° C./min, should be no more than5×10⁻⁶° C.⁻¹ (e.g., no more than 4×10⁻⁶° C.⁻¹, no more than 3×10⁻⁶°C.⁻¹, no more than 2×10⁻⁶° C.⁻¹, no more than 1×10⁻⁶° C.⁻¹, no more than5×10⁻⁷° C.⁻¹, no more than 4×10⁻⁷° C.⁻¹, no more than 3×10⁻⁷° C.⁻¹, nomore than 2×10⁻⁷° C.⁻¹).

While selecting materials having similar TEC's can minimize an elasticstress component, residual stress can also develop from viscoelasticstress components. A viscoelastic stress component arises when there issufficient difference between strain point or glass transitiontemperatures of the component materials. As a material cools below T_(g)it undergoes a sizeable volume contraction. As the viscosity changes inthis transition upon cooling, the time needed to relax stress increasesfrom zero (instantaneous) to minutes. For example, consider a compositepreform made of a glass and a polymer having different glass transitionranges (and different T_(g)'s). During initial drawing, the glass andpolymer behave as viscous fluids and stresses due to drawing strain arerelaxed instantly. After leaving the hottest part of the draw furnace,the fiber rapidly loses heat, causing the viscosities of the fibermaterials to increase exponentially, along with the stress relaxationtime. Upon cooling to its T_(g), the glass and polymer cannotpractically release any more stress since the stress relaxation time hasbecome very large compared with the draw rate. So, assuming thecomponent materials possess different T_(g) values, the first materialto cool to its T_(g) can no longer reduce stress, while the secondmaterial is still above its T_(g) and can release stress developedbetween the materials. Once the second material cools to its T_(g),stresses that arise between the materials can no longer be effectivelyrelaxed. Moreover, at this point the volume contraction of the secondglass is much greater than the volume contraction of the first material(which is now below its T_(g) and behaving as a brittle solid). Such asituation can result sufficient stress buildup between the glass andpolymer so that one or both of the portions mechanically fail. Thisleads us to a third selection criterion for choosing fiber materials: itis desirable to minimize the difference in T_(g)'s of componentmaterials to minimize viscoelastic stress generation in a fiber duringdrawing. Preferably, the glass transition temperature of a firstmaterial, T_(g1), should be within 100° C. of the glass transitiontemperature of a second material, T_(g2) (e.g., |T_(g1)−T_(g2)| shouldbe less than 90° C., less than 80° C., less than 70° C., less than 60°C., less than 50° C., less than 40° C., less than 30° C., less than 20°C., less than 10° C.).

Since there are two mechanisms (i.e., elastic and viscoelastic) todevelop permanent stress in drawn fibers due to differences betweenconstituent materials, these mechanisms may be employed to offset oneanother. For example, materials constituting a fiber may naturallyoffset the stress caused by thermal expansion mismatch if mismatch inthe materials T_(g)'s results in stress of the opposite sign.Conversely, a greater difference in T_(g) between materials isacceptable if the materials' thermal expansion will reduce the overallpermanent stress. One way to assess the combined effect of thermalexpansion and glass transition temperature difference is to compare eachcomponent materials' temperature-length curve. After finding T_(g) foreach material using the foregoing slope-tangent method, one of thecurves is displaced along the ordinate axis such that the curvescoincide at the lower T_(g) temperature value. The difference in y-axisintercepts at room temperature yields the strain, ∈, expected if theglasses were not conjoined. The expected tensile stress, σ, for thematerial showing the greater amount of contraction over the temperaturerange from T_(g) to room temperature, can be computed simply from thefollowing equation:σ=E·∈,where E is the elastic modulus for that material. Typically, residualstress values less than 100 MPa (e.g., less than 50 MPa, less than 30MPa), are sufficiently small to indicate that two materials arecompatible.

A fourth selection criterion is to match the thermal stability ofcandidate materials. A measure of the thermal stability is given by thetemperature interval (T_(x)−T_(g)), where T_(x) is the temperature atthe onset of the crystallization as a material cools slowly enough thateach molecule can find its lowest energy state. Accordingly, acrystalline phase is a more energetically favorable state for a materialthan a glassy phase. However, a material's glassy phase typically hasperformance and/or manufacturing advantages over the crystalline phasewhen it comes to fiber waveguide applications. The closer thecrystallization temperature is to the glass transition temperature, themore likely the material is to crystallize during drawing, which can bedetrimental to the fiber (e.g., by introducing optical inhomogeneitiesinto the fiber, which can increase transmission losses). Usually athermal stability interval, (T_(x)−T_(g)) of at least 80° C. (e.g., atleast 100° C.) is sufficient to permit fiberization of a material bydrawing fiber from a preform. In preferred embodiments, the thermalstability interval is at least 120° C., such as 150° C., 200° C. ormore. T_(x) can be measured using a thermal analysis instrument, such asa differential thermal analyzer (DTA) or a differential scanningcalorimeter (DSC).

A further consideration when selecting materials that can be co-drawnare the materials' melting temperatures, T_(m). At the meltingtemperature, the viscosity of the material becomes too low tosuccessfully maintain precise geometries during the fiber draw process.Accordingly, in preferred embodiments the melting temperature of onematerial is higher than the working temperature of a second,rheologically compatible material. In other words, when heating apreform, the preform reaches a temperature at it can be successfullydrawn before either material in the preform melts.

One example of a pair of materials which can be co-drawn and whichprovide a photonic crystal fiber waveguide with high index contrastbetween layers of the confinement region are As₂Se₃ and the polymer PES.As₂Se₃ has a glass transition temperature (T_(g)) of about 180° C. and athermal expansion coefficient (TEC) of about 24×10⁻/° C. At 10.6 μm,As₂Se₃ has a refractive index of 2.7775, as measured by Hartouni andcoworkers and described in Proc. SPIE, 505, 11 (1984), and an absorptioncoefficient, α, of 5.8 dB/m, as measured by Voigt and Linke anddescribed in “Physics and Applications of Non-Crystalline Semiconductorsin Optoelectronics,” Ed. A. Andriesh and M. Bertolotti, NATO ASI Series,3. High Technology, Vol. 36, p. 155 (1996). Both of these references arehereby incorporated by reference in their entirety. PES has a TEC ofabout 55×10⁻⁶/° C. and has a refractive index of about 1.65.

In some embodiments, photonic crystal fiber waveguides, such aswaveguide 100, can be made by rolling a planar multilayer article into aspiral structure and drawing a fiber from a preform derived from thespiral structure.

Referring to FIG. 2A, to prepare a preform, a glass is deposited 220 ona surface 211 of a polymer film 210. The glass can be deposited bymethods including thermal evaporation, chemical vapor deposition, orsputtering. Referring to FIG. 2B, the deposition process provides amultilayer article 240 composed of a layer 230 of glass on polymer film210.

Referring to FIG. 2C, following the deposition step, multilayer film 240is rolled around a mandrel 255 (e.g., a hollow glass, such as aborosilicate glass, or polymer tube) to form a spiral tube. A number(e.g., about three to ten) of polymer films are then wrapped around thespiral tube to form a preform wrap. In some embodiments, the polymerfilms are made from the same polymer or glass used to form multilayerarticle. Under vacuum, the preform wrap is heated to a temperature abovethe glass transition temperature of the polymer(s) and glass(es) formingmultilayer film 240 and the films wrapped around the spiral tube. Thepreform wrap is heated for sufficient time for the layers of the spiraltube to fuse to each other and for the spiral tube to fuse to polymerfilms wrapped around it. The temperature and length of time of heatingdepends on the preform wrap composition. Where the multilayer iscomposed of As₂Se₃ and PES and the wrapping films are composed of PES,for example, heating for 15-20 minutes (e.g., about 18 minutes) at200-300° C. (e.g., about 250° C.) is typically sufficient. The heatingfuses the various layers to each other, consolidating the spiral tubeand wrapping films. The consolidated structure is shown in FIG. 2D. Thespiral tube consolidates to a multilayer region 260 corresponding torolled multilayer film 240. The wrapped polymer films consolidate to amonolithic support cladding 270. The consolidated structure retains ahollow core 250 of mandrel 255.

As an alternative to wrapping polymer films around the spiral tube toprovide support cladding 270, the spiral tube can be inserted into ahollow tube with inner diameter matching the outer diameter of thespiral tube.

Mandrel 255 is removed from the consolidated structure to provide ahollow preform that is then drawn into a fiber. The preform has the samecomposition and relative dimensions (e.g., core radius to thickness oflayers in the confinement region) of the final fiber. The absolutedimensions of the fiber depend on the draw ratio used. Long lengths offiber can be drawn (e.g., up to thousands of meters). The drawn fibercan then be cut to the desired length.

Preferably, consolidation occurs at temperatures below the glasstransition for the mandrel so that the mandrel provides a rigid supportfor the spiral tube. This ensures that the multilayer film does notcollapse on itself under the vacuum. The mandrel's composition can beselected so that it releases from the innermost layer of the multilayertube after consolidation. Alternatively, where the mandrel adheres tothe innermost layer of the multilayer tube during consolidation, it canbe removed chemically, e.g., by etching. For example, in embodimentswhere the mandrel is a glass capillary tube, it can be etched, e.g.,using hydrofluoric acid, to yield the preform.

In embodiments where a solid core is desired, the multilayer tube can beconsolidated around a solid mandrel that is co-drawn with the otherparts of the fiber. Alternatively, in other embodiments, the multilayerfilm can be rolled without a mandrel to provide a self-supporting spiraltube.

In some embodiments, glass can be coated on both sides of polymer film210. This can be advantageous because the each glass layer only needs tobe half as thick as a glass layer deposited on one side only. Thinnerglass layers are typically less susceptible to mechanical stress damagethat can occur during rolling.

Photonic crystal fiber waveguides prepared using the previouslydiscussed technique can be made with a low defect density. For example,waveguides can have less than about one defect per 10 meters of fiber(e.g., less than about one defect per 20 meters, 50 meters, 100 metersof fiber). Defects include both material defects (e.g., impurities) andstructural defects (e.g., delamination between layers, cracks withlayers), both of which can scatter guided radiation from the coreresulting in signal loss and can cause local heating of the fiber.Accordingly, reducing fiber defects is desirable in applicationssensitive to signal loss (e.g., in high power applications whereradiation absorbed by the fiber can cause damage to the fiber).

Photonic crystal fiber waveguides formed from glass films coated on bothsides of a substrate provide slightly different index profile than thoseformed with a single coated side. Referring to FIGS. 3A and 3B, forexample, a confinement region 310 of a fiber formed from a multilayerfilm coated on both sides has a continuous spiral polymer layer 330 andglass layer 340. The innermost 340A and outermost 340B regions of glasslayer 340 correspond to a single glass layer coated on the polymer,compared to the double layer thickness that occurs for the otherregions. The resulting index profile, taken through a radial section360, is illustrated in FIG. 3B.

In some embodiments, two or more multilayer films can be prepared andstacked before rolling. In this way, the number of layers in theconfinement region can be increased without increasing the size of thefilm.

As discussed previously, photonic crystal fiber waveguides can be usedto guide IR radiation. IR radiation has a wavelength between about 0.7microns and 20 microns (e.g., between about 2 to 5 microns or betweenabout 8 to 12 microns). In some embodiments, photonic crystal fiberwaveguides can be used to guide radiation from IR lasers, such as from aCO₂ laser that emits radiation having a wavelength of about 6.5 micronsor 10.6 microns. Other examples of lasers which can emit IR energyinclude Nd:YAG lasers (e.g., at 1.064 microns) Er:YAG lasers (e.g., at2.94 microns), Er, Cr:YSGG (Erbium, Chromium doped Yttrium ScandiumGallium Garnet) lasers (e.g., at 2.796 microns), Ho:YAG lasers (e.g., at2.1 microns), free electron lasers (e.g., in the 6 to 7 micron range),and quantum cascade lasers (e.g., in the 3 to 5 micron range).

In some embodiments, photonic crystal fiber waveguides can be used toguide radiation with extremely high power densities. For example,waveguides can be used to guide radiation having power densities morethan about 100 W/cm² (e.g., more than about 300 W/cm², 500 W/cm², 1kW/cm², such as about 10 kW/cm² or more). Hollow core waveguides, inparticular, are well suited to such applications due to low absorptionof guided radiation in the core. Absorption losses can be furthermitigated by selecting materials for the waveguide's confine region withlow absorption at the guided wavelengths. As discussed previously,chalcogenide glasses, for example, have low absorption at IR wavelengthsand are well suited for high power IR waveguides. Radiation losses,which not only degrade waveguide performance but can also damage thewaveguide, can also be reduced by selecting materials with high indexcontrast for the confinement region.

High power densities can be generated in fiber waveguides by couplingradiation from high power lasers into the fiber. For example, radiationfrom high power IR lasers, such as those listed above, can be guidedusing photonic crystal fiber waveguides. Laser output power can be morethan about one Watt (e.g., about five Watts, 10 Watts, 25 Watts ormore). In some applications, the laser output energy can be more thanabout 100 Watts, such as several hundred Watts (e.g., more than about200 Watts, 300 Watts, 500 Watts, 1 kilowatt).

In some embodiments, photonic crystal fiber waveguides can haverelatively low transmission losses. For example, transmission losses canbe less than about 2 dB/m (e.g., less than about 1 dB/m, 0.5 dB/m, suchas 0.2 dB/m or less). The fiber waveguides can have low transmissionloss at IR wavelengths, such as for certain wavelengths from about 3-5microns (e.g., about 3.5 microns) or from about 10-12 microns (e.g.,about 10.6 microns). Transmission loss can be substantially lower (e.g.,1-3 or more orders of magnitude smaller) than TIR optical fibers madefrom similar materials. For example, a photonic crystal fiber waveguidehaving a hollow core and chalcogenide glass/polymer confinement regioncan have a substantially lower transmission loss than a TIR fiber with achalcogenide glass core and a polymer cladding. As₂Se₃, for example,reportedly has losses of about 7-10 dB/m at 10.6 microns, while PES haslosses of approximately 100,000 dB/m at 10.6 microns. In contrast, aphotonic crystal fiber waveguide with an As₂Se₃/PES confinement regioncan have losses less than about 1 dB/m. It is believed that thecomparatively low losses are made possible by the short penetrationdepths of guided electromagnetic waves into the confinement region ofthe fiber. Therefore, even though the materials in the confinementregion may have relatively high absorption at the guided wavelengths,the interaction between the guided radiation and the material isminimal.

Photonic crystal fiber waveguides can also experience relatively lowloss due to bends in the fiber. For example, a 90 degree bend in a fiberwith a radius of curvature less than about 10 cm (e.g., less than about5 cm, such as 4 cm or less) can cause losses below about 2 dB (e.g., 1.5dB, 1 dB, 0.5 dB or less). Having relatively low loss associated withbending is advantageous in many fiber applications where the relativestrength of a transmitted signal preferably does not vary substantiallyas the fiber bends during use.

Low transmission loss (e.g., intrinsic loss and/or loss due to bends) isalso typically advantageous in high power applications where power lostalong the length of the fiber can damage the fiber in addition todelivering less power from the radiation source to its destination.

EXAMPLES

A variety of fibers were fabricated by depositing a 5-10 micron thickAs₂Se₃ layer using thermal evaporation onto a 25-50 micron thick PESfilm and subsequently rolling the coated films around hollow glassmandrels. The tubes were clad with a thick outer layer of PES andconsolidated by heating under vacuum. After consolidation, the mandrelswere etched away by introducing hydrofluoric acid into the hollow coreof the mandrel. Etching provided layered preforms, each of which wasplaced in an optical fiber draw tower and drawn into tens to hundreds ofmeters of fiber.

The nominal position(s) of the photonic band gap(s) of each fiber weredetermined by monitoring the outer diameter (OD) of the fiber during thedraw process. The photonic band gap positions were determined from thedraw ratio, provided by the OD measurement. Typical standard deviationsof the fiber OD were approximately one percent of the OD.

Referring to FIGS. 4A and 4B, scanning electron microscope (SEM)analysis of one of the fiber's cross section revealed that the drawnfibers generally maintained proportionate layer thickness ratios andthat the PES and As₂Se₃ films adhered well during the thermal cyclingand elongation associated with the fabrication process. Within themultilayer structure shown in FIGS. 4A and 4B, the PES layers (grey) hada thickness of about 900 nm, and the As₂Se₃ layers (bright) were about270 nm thick (except for the first and last As₂Se₃ layer, which were 135nm thick).

Broadband fiber transmission spectra were measured with a Fouriertransform infrared (FTIR) spectrometer (Nicolet Magna 860), using aparabolic mirror to couple light into the fiber and an externaldetector. The results of these measurements are shown in FIG. 5 forfibers having two different layer structures. For each spectrum, lightwas guided at the fundamental and high-order photonic band gaps.

Some fibers were prepared having hollow core diameters of 700-750microns and ODs of 1300-1400 microns with a fundamental photonic bandgap spanning the 10-11 micron wavelength regime. FIG. 6A shows an FTIRtransmission spectrum for one of these fibers, measured using anapproximately 30 cm long straight section of fiber.

In order to quantify the transmission losses in these fibers, fibercutback measurements were performed. These measurements involvedcomparing the intensity of radiation transmitted through about 4 metersof straight fiber with the intensity of transmission through the samesection of fiber cut to shorter lengths (see FIG. 6B). This test wasperformed on multiple sections of fiber, and the results were found tobe nearly identical for the different sections tested. The measurementswere performed using a 25 Watt CO₂ laser (GEM-25, Coherent-DEOS) andhigh power detectors (818T-10 detectors, obtained from Newport). Thefiber was held straight, secured at both ends as well as at multiplepoints between the fiber ends to reduce variations in the input couplingand propagation conditions during fiber cutting. The laser beam wasdirected through focusing lenses as well as a 500 micron diameterpinhole aperture prior to entering the fiber. In addition, the input endface of the fiber was coated with a metal film to reduce accidentallaser damage from misalignment.

Transmission losses in the fibers' fundamental band gap at about 10.6microns were measured to be about 0.95 dB/m, as shown in FIG. 6B, withan estimated measurement uncertainty of about 0.15 dB/m. A bendinganalysis (discussed below) for fibers with a band gap centered at about10.6 microns revealed bending losses below about 1.5 dB/m for 90 degreebends with bending radii from 4-10 cm.

Bend loss measurements were performed using a broad band FTIR source anda CO₂ laser operating at 10.6 microns. For each measurement, the fiberswere bent at an angle of 90 degrees around metal cylinders of varyingradii. The amount of fiber after the bend was about 15 cm in each case.This portion was held straight in each case. FIG. 7 shows the relativeintensities as measured using an FTIR spectrometer for a straight fiberapproximately 50 cm in length and the same fiber at different bendingradii.

The FTIR bending measurements shown in FIG. 7 give a total bending lossvalue below 1 dB for the bend with the largest radius. To corroboratethese results, similar tests were performed with CO₂ laser measurementsusing a length of fiber about 2.5 m long. FIG. 8 shows the averagebending loss in dB for 90 degree bends for the CO₂ laser referenced tothe length of fiber without bends.

The CO₂ laser bending loss results represented an average of multipletrials, and variability observed in the losses was on the order of about0.2 dB. The results obtained using the CO₂ laser had the samequalitative characteristics as those obtained using the FTIR apparatus.It was noted that the different sources used had different coherency,numerical aperture, and polarization state. It was thus expected thatthey would couple to modes having different loss characteristics.

The maximum laser power density coupled into the fibers from the CO₂laser was approximately 300 W/m², which was sufficient to burn holesthrough paper and PES films (the fiber majority component). No damage tothe fibers was observed when the radiation was properly coupled into thehollow fiber core. The CO₂ laser (GEM-25, Coherent-DEOS) was alignedusing a HeNe laser. The HeNe laser was used to trace the path of the CO₂laser, allowing the apparatus to be aligned with a relatively low powerlaser. A ZnSe beamsplitter was placed between the laser and the fiber tosplit off a reference beam. The beam transmitted by the beamsplitter wasfocused through a pinhole aperture using a lens assembly prior tocoupling into the fiber. Data was collected simultaneously from thebeamsplitter reference beam and the fiber output using a Newportdual-channel power meter with a GPIB/Labview computer interface.

In the cutback measurements, the fiber was cut using a swiftrazor-cutting action, which produced reasonably reproducible cuts. Inorder to account for any residual variability in cutting, any short cuts(e.g., 1-2 mm) were performed around each data point recorded in thecutback measurement data shown in FIG. 6B. Power levels did not varygreatly due to the cuts, although data from obviously poor cuts wasdiscarded. The data measured for each cut at a cut length was averagedto provide the data shown in FIG. 6B. In addition, the power ratiobetween the transmitted and reference beams measured after each cut wastime-averaged over several minutes.

Variable modal output patterns were also observed in fibers by varyingthe input coupling conditions of the CO₂ laser in approximately 4 meterslong fibers with core diameters of about 700 microns that were heldnominally straight. The modal output patterns were observed by imagingthe output beam using a Spiricon Pyrocam III. Imaging different modalpatterns suggested that the fiber operates in a relatively few-moderegime (e.g., the fibers have about 10 or fewer guided modes).

ADDITIONAL EMBODIMENTS

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. A method for forming a fiber waveguide, themethod comprising: rolling a multilayer film into a structure having aspiral cross-section, wherein the multilayer film comprises at least twolayers of materials with different refractive indices at a wavelength λ,the materials being thermomechanically compatible with each other; andforming a fiber waveguide from the structure, the fiber waveguide havinga core extending along a waveguide axis and a portion surrounding thecore having a spiral cross-section, the refractive index of thematerials and a thickness of each of the layers having values such thatthe portion of the fiber waveguide having the spiral cross-section isstructured to guide radiation at λ in the core along the waveguide axis,wherein the forming the fiber waveguide comprises drawing a fiberpreform derived from the structure having the spiral cross-section. 2.The method of claim 1, wherein the layers comprise a layer of a firstmaterial and a pair of layers of a second material sandwiching the firstmaterial layer.
 3. The method of claim 1, wherein the differentmaterials comprise a first material comprising a glass and a secondmaterial comprising a polymer.
 4. The method of claim 1, furthercomprising: disposing at least a first layer of a first material on asecond layer of a second material different from the first material toform the multilayer film.
 5. The method of claim 4, wherein the firstmaterial is disposed on both sides of the second layer.
 6. The method ofclaim 4, wherein the second material is a polymer.
 7. The method ofclaim 6, wherein the polymer comprises PES or PEI.
 8. The method ofclaim 4, wherein the disposing comprises sputtering.
 9. The method ofclaim 4, wherein the disposing comprises evaporating.
 10. The method ofclaim 4, wherein additional layers are disposed on the first and secondlayers to form the multilayer film.
 11. The method of claim 4, whereinthe first material is a glass.
 12. The method of claim 11, wherein theglass is a chalcogenide glass.
 13. The method of claim 1, wherein themultilayer film is rolled around a rod to form the structure having thespiral cross-section.
 14. The method of claim 13, wherein the rod ishollow.
 15. The method of claim 13, further comprising consolidating thestructure having the spiral cross-section to form the preform.
 16. Themethod of claim 15, wherein the consolidating comprises heating thestructure having the spiral cross-section.
 17. The method of claim 16,wherein the consolidating comprises heating the structure having thespiral cross-section under vacuum.
 18. The method of claim 13, furthercomprising removing the rod from the preform prior to the drawing. 19.The method of claim 18, wherein the rod is removed by chemicallyetching.
 20. The method of claim 1, wherein the structure having thespiral cross-section comprises a core surrounded by the multilayer film.21. The method of claim 1, where the core is a hollow core and theportion having the spiral cross-section comprises multiple layerscorresponding to the multilayer film.
 22. The method of claim 1, whereinthe multilayer film comprises a polymer film and a layer of a glassdeposited on the polymer film.
 23. The method of claim 1, wherein themultilayer film comprises a pair of amorphous materials that havesubstantially different refractive indices.
 24. The method of claim 23,wherein one of the amorphous materials is an inorganic glass.
 25. Themethod of claim 24, wherein the other one of the amorphous materials isa thermoplastic polymer.
 26. The method of claim 24, wherein theinorganic glass is a chalcogenide glass.
 27. The method of claim 26,wherein the chalcogenide glass is As₂Se₃.
 28. The method of claim 27,wherein the thermoplastic polymer is polyethersulfone or polyetherimide.